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Modelling the turbulent diffusion of thermal energy, momentum, and metals isrequired in all galaxy evolution simulations due to the ubiquity of turbulencein galactic environments. The most commonly employed diffusion model, theSmagorinsky model, is known to be over-diffusive due to its strong dependenceon the fluid velocity shear. We present a method for dynamically calculating amore accurate, locally appropriate, turbulent diffusivity: the dynamiclocalised Smagorinsky model. We investigate a set of standardastrophysically-relevant hydrodynamical tests, and demonstrate that the dynamicmodel curbs over-diffusion in non-turbulent shear flows and improves thedensity contrast in our driven turbulence experiments. In galactic discs, wefind that the dynamic model maintains the stability of the disc by preventingexcessive angular momentum transport, and increases the metal-mixing timescalein the interstellar medium. In both our isolated Milky Way-like galaxies andcosmological simulations, we find that the interstellar and circumgalacticmedia are particularly sensitive to the treatment of turbulent diffusion. Wealso examined the global gas enrichment fractions in our cosmologicalsimulations, to gauge the potential effect on the formation sites andpopulation statistics of Population III stars and supermassive black holes,since they are theorised to be sensitive to the metallicity of the gas out ofwhich they form. The dynamic model is, however, not for galaxy evolutionstudies only. It can be applied to all astrophysical hydrodynamics simulations,including those modelling stellar interiors, planetary formation, and starformation.

Dynamic Localised Turbulent Diffusion and its Impact on the Galactic Ecosystem